Chemical vapor deposition (CVD) methods are employed to form films of material onto substrates such as wafers or other surfaces during the manufacture or processing of semiconductors. In CVD, a CVD precursor, also known as a CVD chemical compound, is decomposed thermally, chemically, photochemically or by plasma activation, to form a thin film having a desired composition. For instance, a vapor phase CVD precursor can be contacted with a substrate that is heated to a temperature higher than the decomposition temperature of the precursor, to form a metal or metal oxide film on the substrate.
Preferably, CVD precursors are volatile, heat decomposable and capable of producing uniform films under suitable CVD conditions. In producing thin films by CVD processes, precursors that are liquid at room temperature, rather than solid, often are preferred.
Thin films that include ruthenium (Ru) or ruthenium oxide (RuO2), for instance, have good electrical conductivity, high work function, are chemically and thermally stable, resistant to inter-layer chemical diffusion and are compatible with many dielectric substrate materials. Ru and RuO2 films, for instance, have been investigated as film electrode material for semiconductor devices such as DRAM (Dynamic Random Access Memory) devices.
Bis(pentahaptocyclopentadienyl)ruthenium (ruthenocene) and the symmetrical, diethyl-substituted ruthenocene (1,1′-diethylruthenocene) have been investigated as possible precursors for forming ruthenium-based thin films by CVD techniques. Both have relatively low vapor pressure (less than 10 Torr at 100° C.). At room temperature, ruthenocene is a solid and 1,1′-diethylruthenocene is a liquid.
To date, ruthenocene or symmetrically substituted 1,1′-diethylruthenocene have been prepared by several synthetic routes.
One existing method for forming ruthenocene includes the reaction of RuCl3.XH2O with cyclopentadiene, in the presence of Zn, to produce ruthenocene, ZnCl2 and HCl, as shown in FIG. 1A. A similar approach, using ethyl-substituted cyclopentadiene, has been employed to produce 1,1′-diethylruthenocene, as shown in FIG. 1B. Generally, yields obtained by this method are about 70%.
As shown in FIG. 1C, unsubstituted ruthenocene also has been prepared by the reaction of cyclopentadiene, chloro(cyclopentadienyl)bis(triphenylphosphine)ruthenium(II) and sodium hydride (NaH) in benzene. Chloro(cyclopentadienyl)bis(triphenylphosphine)ruthenium(II) precursor has been synthesized by reacting ruthenium trichloride and triphenylphosphine in ethanol.
Another method that has been investigated for the synthesis of ruthenocene includes the transmetallation reactions of a bis(alkylcyclopentadienyl)iron compound with RuCl3.XH2O and results in the formation of low yield 1,1′-dialkylruthenocene, iron trichloride (FeCl3) and difficult to separate iron species.
Monosubstituted ruthenocenes, e.g., 1-ethylruthenocene, are formed as an impurity during the synthesis of 1,1′-diethylruthenocene. Tert-butyl(cyclopentadienyl)(cyclopentadienyl)ruthenium also has been prepared by reacting a heated mixture of bis(cyclopentadienyl)ruthenium, aluminum chloride and polyphosphoric acid, with tert-butyl alcohol, followed by distillation.
Generally, synthetic methods described above often are associated with low yields, competing dimerization reactions, require complex product separations, and/or special handling techniques. NaH, for instance, reacts violently with H2O and also reacts with air.
Furthermore, synthetic approaches shown in FIGS. 1A and 1B include a one step addition of both cycolpentadienyl rings and thus are suitable for preparing unsubstituted ruthenocene or the symmetrically substituted 1,1′-diethylruthenocene, but not asymmetric, di-substituted ruthenocenes, such as 1-D1,1′-D1′-ruthenocenes, where D1 and D1′ are different alkyl groups.
In developing methods for forming thin films by CVD methods, a need continues to exist for methods of preparing precursors that preferably are liquid at room temperature, have relatively high vapor pressure and that can form uniform films. A need also continues to exist for versatile methods of preparing ruthenocenes and other Group 8 (VIII) metallocenes. In particular, a need exists for methods for preparing di- or multi-substituted asymmetric Group 8 (VIII) metallocenes